1. Field of the Invention
The present invention relates to a mass spectrometry apparatus, and more particularly to a mass spectrometry apparatus able to simultaneously or separately measure the molecular weight and analyze the molecular structure of a detected gas with a sufficient sensitivity.
2. Description of the Related Art
Measurement of the mass of gas molecules, which are electrically neutral, requires ionization of the gas molecules. The apparatus for ionizing the gas is called an xe2x80x9cion sourcexe2x80x9d. The ionized molecules (hereinafter referred to as xe2x80x9cionsxe2x80x9d) enter the mass spectrometry mechanism. In the mass spectrometry mechanism, the ions are led into a specific electric field or magnetic field and move along a path in accordance with the mass of the ions due to the electrical field or magnetic field applied to the ions. As a result, different paths arise for each ion and only ions of a specific mass are detected.
Various ionization systems have been proposed in the past for the above ion source. For example, (1) electron impact ionization, (2) chemical ionization, (3) ionization by a composite of electron impact and chemical ionization, (4) atmospheric pressure ionization, (5) ionization by a composite of electron impact and atmospheric pressure ionization, and (6) ionization by ion attachment have been proposed. These ionization methods will be explained in brief below.
(1) Electron Impact Ionization
The electron impact ionization method is the method used most often for the ion sources of mass spectrometry apparatuses. In the ion source of the electron impact ionization method, the detected gas is introduced in an amount of 10xe2x88x923 Pa and the molecules of the detected gas are impacted by hot electrons emitted from a hot filament to be accelerated to about 50 to 100 eV. The negative charge electrons are stripped from the gas molecules by the electron impact, whereby the gas molecules become positive charge ions. The electron impact ionization method is simple in terms of hardware and has the advantage of a small difference in ionization efficiencies resulting from the type of the molecules. The pressure in the ion source is usually 10xe2x88x922 Pa or less, so as not to limit the movement of the electrons and ions. Note that the pressure in the mass spectrometry mechanism is usually 10xe2x88x923 Pa or less. In the ion trap type, however, operation is possible even with 10xe2x88x922 Pa.
The above electron impact ionization method has the feature of splitting (dissociating) molecules along with the ionization due to the excess energy of the electron impact when applied to a detected gas comprised of molecules with small energy of atomic bonds. Therefore, in this case, there is the advantage of obtaining effective information on the molecular structure. On the reverse side, there is the defect that it is not possible to obtain effective information on the molecular weight.
(2) Chemical Ionization
In an ion source of the chemical ionization method, a reaction gas of about 100 Pa (methane: CH4 etc.) and a detected gas of about 1 Pa are introduced. First, the reaction gas is ionized by the electron emission and impact from the hot filament. Next, an ion and molecular reaction occurs between the ionized reaction gas and the detected gas. The detected gas is ionized to a positive charge or negative charge. The mechanism of ionization is extremely complicated and includes the phenomena of 1) the hydrogen ions in the ions of the reaction gas bonding with the molecules of the specimen, 2) hydrogen ions conversely being stripped from the detected gas, 3) charge movement, etc. Even in the chemical ionization method, when the hydrogen ions bond with the specimen molecules, disassociation often occurs in a detected gas with a weak bond energy. The chemical ionization method has the defect of a poor stability and reproducibility of the measurement values due to the complicated ionization mechanism. Further, it has the defect of a low measurement sensitivity.
(3) Ionization by Composite of Electron Impact and Chemical Ionization
There are two types of ion sources in this ionization method as shown in FIG. 11A and FIG. 11B and in FIG. 12. The configuration of FIG. 11A and FIG. 11B is a switching type. The configuration of FIG. 12 is a continuous type.
According to the switching type ion source shown in FIG. 11A and FIG. 11B, one of the electron impact ionization method (FIG. 11A) and the chemical ionization method (FIG. 11B) is selected and used by mechanical and electrical switching. FIG. 11A shows the state of operation in the case of electron impact ionization. An ion source including a filament 101 and a region 102 for electron impact and a condensing lens 104 are arranged in the same space 105. This space 105 is evacuated by a single vacuum pump 106. A carrier gas of He and the detected gas (specimen) are introduced to give a pressure of about 10xe2x88x923 Pa. A partition 109 with an ion passage port 108 for passing the produced and condensed ions is provided at the front of the mass spectrometry mechanism 107. The mass spectrometry mechanism 107 is evacuated by another vacuum pump 110 so as to maintain the pressure of 10xe2x88x924 Pa. FIG. 11B shows the state of operation in the case of chemical ionization. The filament 101 and the condensing lens 107 remain unchanged, but the electron impact region 102 where the electrons impact is generally surrounded by the partition 111. The electron impact region 102, however, also has an opening 112 such as the electron passage port. This does not mean that ports other than the ion passage port 113 are closed. The electron impact region 102 is evacuated by the vacuum pump 106 through the space 105 where the filament 101 and the condensing lens 104 are positioned. A reaction gas (CH4) and a detected gas (specimen) are introduced into the electron impact region 102 to a pressure of 100 Pa. The ratio of the detected gas, however, is about 1%. Note that the pressure in the space around the electron impact region becomes 10xe2x88x922 Pa. In the above composite method, there is the defect of the need for switching of the ion source itself and the introduced gas and poor operability. Therefore, while this system is possible in current GC/MS products, in almost all cases only the electron impact ionization method is used. The chemical ionization method is only used on a supplementary basis.
The continuous type ion source shown in FIG. 12 has a special structure designed especially for research (Analytical Chemistry, vol. 43, no. 12 (1971), p. 1720). In this structure, it is possible to perform the electron impact ionization and the chemical ionization continuously or simultaneously. Exclusive filaments 201 and 202 for the ionization methods are provided. The electron impact regions 203 and 204 are also independent. There is however no condensing lens. The electron impact region 204 of the chemical ionization method is substantially surrounded by the partition 205. The ion source of the electron impact ionization method is positioned in the space around the electron impact region 204 of the chemical ionization method. These are evacuated by a single vacuum pump 206. A reaction gas and a 1% detected gas are introduced to give a pressure of 100 Pa in the electron impact region 205 of the chemical ionization method. The reaction gas and the detected gas are reduced in pressure 10xe2x88x924 fold, that is, to about 10xe2x88x922 Pa, while leaving the ratio the same, and flow to the ion source of the electron impact ionization method. Therefore, the partial pressure of the detected gas at the ion source of the electron impact ionization method becomes a low 10xe2x88x924 Pa or so. The above composite system has the defects that not only does it ionize the reaction gas by the electron impact ionization method, but also the concentration of the detected gas is low and the sensitivity is poor. Therefore, products of this system have still not been commercialized. Note that in FIG. 12, elements substantially the same as elements explained with reference to FIG. 11A and FIG. 11B are given the same reference numerals and explanations thereof are omitted.
(4) Atmospheric Pressure Ionization
In this ion source, the carrier gas and a slight amount of the detected gas are introduced at atmospheric pressure (1xc3x97105 Pa) and the sensitivity is improved. Since it has the following ionization mechanism, if the ratio of the detected gas to the carrier gas is 1% or less, the feature of the high sensitivity of this system does not appear. As the carrier gas, He, Ar, or another gas with a large ionization potential is selected. As the ionization mechanism, first, the carrier gas is ionized by the corona discharge from needle-shaped electrodes. Next, due to the exchange of charges between the ionized carrier gas and detected gas, electrons are stripped from the detected gas and the detected gas is ionized to a positive charge. Due to the exchange of charges from the carrier gas of the main ingredient, even if there is only a slight amount of the detected gas, the ratio of the ionized detected gas becomes high and as a result high sensitivity measurement becomes possible.
(5) Ionization by Composite of Electron Impact and Atmospheric Pressure Ionization
This ion source is designed to make up for the defects of electron impact ionization and atmospheric pressure ionization and is a composite of the two systems as shown in FIG. 13. For example, there is the apparatus disclosed in Japanese Examined Patent Publication (Kokoku) No. 56-21096. The ion source 301 of the electron impact ionization method (EI) is positioned at the front of the mass spectrometry mechanism 302. These are evacuated by a single vacuum pump 303. There are two partitions 305 and 306 between the ion source 301 of the electron impact ionization method and the ion source 304 of the atmospheric pressure ionization (API). The center space is evacuated by a separate vacuum pump 307. A carrier gas (Ar) of the atmospheric pressure (1xc3x97105 Pa) and the detected gas (specimen) are introduced into the ion source 304 of the atmospheric pressure ionization. The detected gas is introduced in a slight amount, for example, 0.1%, that is, about 100 Pa. The carrier gas and the detected gas are reduced in pressure 10xe2x88x928 fold, that is, to about 10xe2x88x923 Pa, through the two partitions 305 and 306, and flow into the ion source 301 of the electron impact ionization method. Therefore, the detected gas at the ion source 301 of the electron impact ionization method is raised in concentration by the design of the shape of the partitions, but the partial pressure is supposed to fall to about 10xe2x88x925 Pa. In the above composite system, there are the same defects as the above-mentioned composite system of electron impact and chemical ionization. In the electron impact ionization method, not only is the carrier gas ionized, but also the concentration of the detected gas is low, so the sensitivity is poor. Therefore, even in products using this system, the electron impact ionization method is only used on a supplementary basis.
(6) Ionization by Ion Attachment
This ionization system makes use of the phenomenon that when an oxide of an alkali metal is heated, positive charge metal ions are emitted from the surface in the form of Li+ or Na+ etc. This ionization system includes the following three main methods:
The first method, as described by Hodge, Analytical Chemistry, vol. 48, no. 6, p. 825 (1976), obtains metal ions from an emitter comprised of a spherical alkali metal oxide attached to a filament and attaches them to the gas molecules for ionization. This method makes use of the phenomenon of gentle or soft attachment of ions to locations of concentrated charges in the gas molecules. The attachment energy is an extremely small one of about 0.43 to 1.30 eV/molecule. There is therefore less occurrence of disassociation. Further, the molecules as a whole are ionized by the attachment of ions. Note that this method is an indirect attachment method because the reaction gas (hydrocarbons etc.) and detected gas are introduced to the ion source, Li+ is attached to the reaction gas once, then Li+ is moved to the molecules of the detected gas.
The second method, as described by Bombick, Analytical Chemistry, vol. 56, no. 3, p. 396 (1984), is a direct attachment system which introduces only the detected gas to the ion source and causes Li+ to directly attach to the molecules of the detected gas. Bombick simultaneously alludes to an apparatus combining the ion attachment ionization and electron impact ionization methods. In this composite apparatus, the emitter is inserted into the electron impact region of the chemical ionization method. The region where the ions are attached is generally surrounded by a partition and is evacuated by a vacuum pump through the space surrounding the position of the condensing lens. The pressure becomes about 10 Pa depending on the detected gas. When operating in the chemical ionization mode, the emitter is given a plus potential of about 5V, then the alkali metal oxide is heated to 5 to 600xc2x0 C. to cause the emission of alkali metal ions. When operating in the electron impact ionization mode, the emitter is given a minus potential of xe2x88x9270V, then the filament is heated to about 1800xc2x0 C. to cause the emission of hot electrons.
The third method, as described by Fujii, Analytical Chemistry, vol. 64, no. 7, p. 775 (1992), Journal of Applied Physics, vol. 82, no. 5, p. 2095 (1997), etc., improves on the above method from the viewpoint of the detection of molecule peaks (no disassociation) and the measurement sensitivity and enables the examination of the limit of the measurement sensitivity and the measurement of extremely unstable radicals together with a plasma apparatus. FIG. 14 briefly shows the hardware configuration. In this apparatus, the ion source 403 is sealed by a partition 402 having no openings other than the ion passage port 401. There are two partitions 405 and 406 up to the mass spectrometry mechanism 404. The spaces 407, 408 and 409 are evacuated independently by pumps 410, 411 and 412. Therefore, even if the pressure of the ion source is made about 100 Pa and the ion passage ports of the partitions are made slightly larger, there is no problem in the operation of the mass spectrometry mechanism 404. In the ion attachment ionization method, due to the low attachment energy, if the excess energy is left as it is, there is a high possibility of the ions again separating from the molecules and disassociation occurring. To prevent this, the ion source is designed to make the pressure a relatively high one of about 100 Pa and quickly absorb the excess energy by impact with the gas. Further, to reduce the reaction (disassociation and cluster) at the emitter surface, the introduced gas is made mainly N2 and the concentration of the detected gas is lowered. Note that metal ions do not easily attach to N2, so this method is a direct attachment method.
Next, a brief explanation will be given of the mass spectrometry mechanism. The mass spectrometry mechanism acts to separate and detect the ion molecules by mass by an electric field and a magnetic field. The most general type of mass spectrometry mechanism is the quadrapole type mechanism called a xe2x80x9cmass filterxe2x80x9d. In the quadrapole type mechanism, a unique quadrapole electric field is formed in the diametrical direction within four poles arranged in parallel to which a voltage with a superposed high frequency and direct current is applied. Only specific ions are stably oscillated. There is uniform (drift) motion in the axial direction, however, so only the specific ions pass through the quadrapole mechanism, are detected at the collector, and are taken out as a signal. The other unstably oscillated ions are absorbed by the electrodes part way.
Recently, a three-dimensional mass spectrometry mechanism (that is, ion trap type mechanism) using a principle similar to that of a quadrapole mechanism has been used as a new type of mass spectrometry mechanism. An ion trap mechanism is comprised of a single donut-shaped ring electrode and two hill-shaped end cap electrodes positioned on its axis. A high frequency voltage is applied to the ring electrode, while a direct current voltage is applied to the end cap electrodes. Due to this, an axially symmetric quadrapole electric field is formed inside. Mass spectrometry is possible at the same location without a drift motion. In the mass spectrometry, first, all ions are trapped, then specific ions are detected through the holes made in the end cap electrodes on the center axis as an unstable motion. This sequence is repeated.
In the above ion trap type mechanism, there are two types of structures relating the position of the ion source. The first is the general type for mass spectrometry apparatuses. This is an external ionization structure where the ion source is outside of the mass spectrometry mechanism and the ions are introduced into the mass spectrometry mechanism from the outside. The other is unique to ion trap types. This is an internal ionization structure where ionization takes place during the mass spectrometry. In the internal ionization structure, electrodes are directly driven into the space formed by the quadrapole electric field (case of electron impact type) and the ionization and mass spectrometry are performed at the same location. The filament is positioned at the side opposite to the detector. The electrons pass through the holes formed in the end cap electrodes on the center axis. Therefore, the second ion source is comprised by a filament, holes, and an impact region of the same space as the analysis region and can be said to be a composite of the mass spectrometry mechanism. Another feature of the ion trap type is that the operable voltage is higher by one order than other mass spectrometry mechanisms. It can also operate at 10xe2x88x922 Pa. In the case of a light gas such as He, it is known that operation is also possible at 10xe2x88x921 Pa and the resolution and sensitivity are conversely improved. As a detected gas, however, 10xe2x88x922 Pa is supposed to be the de facto limit.
In recent years, in gas analysis using the practical mass spectrometry apparatuses, it has been demanded to accurately measure gas molecules with a sufficient sensitivity and further analyze the molecular structure with a sufficient sensitivity. According to the various ionization methods described above, however, it was not possible to satisfy this demand as shown in Table 1.
Further, even in apparatuses combining the conventional ionization methods, there were problems as shown in Table 2.
Here, a more detailed explanation will be given of the problems of the composite system of Bombick ion attachment and electron impact ionization. In this system, the two ionizations are performed using the same filament, so it is impossible to simultaneously measure the molecular weight and analyze the structure. Even when switching, it is necessary to change the settings of the pressure of the detected gas or the power and voltage of the filament, so the operability is poor. Further, under the electron emission conditions, the temperature of the alkali metal oxide becomes considerably high, a large amount of alkali metal is emitted, and the problems of contamination or lifetime become serious. Further, since the region of attachment of ions is sealed, the detected gas to which the metal ions are attached cannot be efficiently drawn out and the sensitivity of the ion attachment ionization method is low. Further, since there is an alkali metal oxide with a large heat capacity at the center of the filament, electrons are not emitted from there and the sensitivity of the electron impact ionization method also becomes low. Further, while the reasons are not necessarily clear, if the voltage of the emitter is made to be more than 5V in the ion attachment ionization method, the molecule peaks are reduced and fragment peaks appear. Therefore, the reliability at the measurement of the molecular weight becomes extremely low.
An object of the present invention is to provide a mass spectrometry apparatus, which can accurately measure the molecular weight of molecules of a detected gas with a sufficient sensitivity and preferably simultaneously analyze the molecular structure with a sufficient sensitivity.
The mass spectrometry apparatus according to the present invention is configured as follows to achieve the above object.
The mass spectrometry apparatus according to the present invention is provided with an ion source for ionizing a detected gas, a mass spectrometry mechanism for analysis of the mass of the ionized detected gas, and a vacuum pump for evacuating these. The mass spectrometry apparatus is further provided independently with two independent ion sources, that is, a first ion source for attaching positive charge metal ions for ionization and a second ion source for causing electrons to impact for ionization. The molecular weight of the detected gas is measured accurately and with a high sensitivity by analyzing the ions of the detected gas produced by the first ion source based on the ion attachment ionization at the mass spectrometry mechanism. Further, the molecular structure is analyzed with a high sensitivity by analyzing the ions of the detected gas produced at the second ion source based on the electron impact ionization at the mass spectrometry mechanism. Due to the above configuration, it is possible to measure the molecular weight of the detected gas and analyze the molecular structure simultaneously or separately with high sensitivities.
In the above configuration, preferably the second ion source is positioned between the first ion source and the mass spectrometry mechanism and the detected gas is introduced to the first ion source.
In the above configuration, a quadrapole or ion trap type mechanism is used for the mass spectrometry mechanism. An ion trap mass spectrometry mechanism has an internal ionization structure comprised combined with the second ion source. As the detected gas introduced to the first ion source, there is a gas ejected from a gas chromatograph or a liquid chromatograph. Further, the metal ions are any of Li+, K+, Na+, Rb+, Cs+, Al+, Ga+, and In+.
The mass spectrometry apparatus according to the present invention is realized by improving the above conventional ion attachment ionization method. In the past, it had been considered that ion attachment ionization could only be applied to specific types of measurement for research purposes, but the inventor discovered that the ion attachment ionization method can be broadly used for general, practical analysis of gas and other mass spectrometry. When using a mass spectrometry apparatus using the ion attachment ionization method of the present invention for actual measurement of the easily disassociable acetone or C4F8, only complete molecule peaks appear and no fragments can be observed at all. Further, since the pressure of the ion source is three orders lower than the atmospheric pressure ionization method, there is almost no occurrence of clusters. Further, depending on the specimen, it was learned that it is not necessarily essential to lower the concentration of the detected gas.
The mass spectrometry apparatus according to the present invention can give a good measurement sensitivity as explained above. C4F8 is a gas extremely often used as an industrial use gas for semiconductors etc. A look by molecular characteristics shows that the polarity is low (little concentration of charges) and the electron affinity is large (negative charge electrons are strongly attracted). Therefore, it had been thought that the positive charge ions would not easily attach and that a sufficient sensitivity could not be obtained by the ion attachment ionization method. When using the ion attachment ionization apparatus of the present invention to actually measure C4F8, however, a sufficient sensitivity of the ppb level was obtained. Further, a sensitivity of the ppm level was actually obtained for non-polar N2, for which the sensitivity should be the worst.
The reason why the high sensitivity is achieved by the mass spectrometry apparatus according to the present invention is believed to be as follows. In the electron impact ionization method or other methods, there is a large background due to the light emitted from the ion source. Further, the background increases in proportion to the amount of the signal. Therefore, it is not possible to sufficiently increase the sensitivity in practice. As opposed to this, with the ion attachment ionization method, since the emitter temperature is low, there is almost no background due to the radiated light. Therefore, the increase in the signal due to the improvement in the ion source contributes to the improvement of the sensitivity as it is.
Due to the above, according to the ion attachment ionization method newly developed by the present invention, it becomes possible to realize a mass spectrometry apparatus for measuring the accurate molecular weight of the gas molecules with a sufficient sensitivity.
Further, the mass spectrometry apparatus according to the present invention can analyze the molecular structure as explained below.
In the past, the simplest, most reliable way to analyze the molecular structure is to use the electron impact ionization method. The electron impact ionization method is therefore combined inside the apparatus for the ion attachment ionization method. As explained above, however, a conventional apparatus combining the electron impact ionization method with the chemical ionization method or atmospheric pressure ionization method was always low in operability or sensitivity. As opposed to this, the inventor discovered the following three advantages obtained by combining the electron impact ionization method with the ion attachment ionization method.
First, it is possible to introduce 100% detected gas in the ion source. That is, there is no need to dilute it with a reaction gas or carrier gas etc.
Second, the pressure of the ion source becomes a low pressure of 100 Pa. That is, the pressure of the ion source is 10xe2x88x923 times in comparison with the atmospheric pressure ionization method.
Third, it is possible to close off the ion source except for the ion passage port. As opposed to this, an electron passage port is required in the chemical ionization method.
By utilizing the advantages of the ion attachment ionization method, a combination with the electron impact ionization method which solves the above problems can be achieved.